For metallic parts (for example, an exterior cover for a gas sensor or the like) to be installed in a car (which will be hereinafter referred to as on-vehicle), generally, stainless steel is used (for example, see Patent Document 1 and Patent Document 2). The Patent Document 1 discloses ferrite based stainless steel which is used in some parts for a car including an oxygen sensor or the like and is excellent in cold workability. The Patent Document 2 discloses a method for fabricating metallic fittings to be used in a sensor for a car by forging and molding of ferrite based stainless steel.
Moreover, on various parts which are mass-produced industrially, product-identifying information, for example, a name of a manufacturer, a name of a country, a product model, a lot number, an individual identification number and the like are generally marked (printed) in a manufacturing process thereof. This is to ensure traceability at a subsequent step or after they are put on the market. As a way of the marking, there is a technique for marking with irradiation of a laser beam. There has already been known the technique for irradiating a surface of a glass base for a liquid crystal with a laser beam, thereby forming a concave portion through ablation (for example, see Patent Document 3). Moreover, there is also known the technique for marking by providing a marking layer which is a thin film metal layer on a surface of an electronic device formed of ceramic and then forming a concave portion on the marking layer by a laser beam (for example, see Patent Document 4).
An on-vehicle gas sensor such as an NOx sensor or an oxygen sensor usually has a structure in which an element body portion is covered with an exterior cover (a protective cover) which is a protective member thereof and is used in a state in which a part thereof is protruded into a piping path for an exhaust gas to be discharged from an engine of a car. The exterior cover is fabricated through pressing where stainless steel having been subjected to a heat treatment in order to ensure the hardness is used as its material.
With respect to the exterior cover also, for the purpose described above, a marking of the product-identifying information with a laser beam is performed. It is necessary to carry out the marking in such a manner that the product-identifying information is visually recognized clearly also after the on-vehicle gas sensor is used on the market. In the case that there is applied a comparatively simple marking technique to irradiate a laser beam to form the concave portion as disclosed in the Patent Document 3, however, there is a problem in that the exterior cover after the marking has a low corrosion resistance and stress corrosion cracking is apt to occur. Specifically, the inventors of the present invention has confirmed that, in a stress corrosion cracking test which is carried out by immersing the exterior cover after the marking in a magnesium chloride solution, stress corrosion cracking whose starting point is a marking portion is apt to occur. This is supposed to be caused because, in order to provide a marking pattern having a visibility on a surface of an exterior cover which originally includes concavo-convex to some extent by fabrication through pressing (deep drawing), the irradiation of a laser beam having a high intensity is carried out to form a deeper concave portion than a surface concavo-convex; whereby a residual stress difference (or a residual stress gradient) between a laser beam irradiated region (a marking region) and a periphery is increased. Roughly speaking, the visibility herein referred is assumed to mean that a target portion and a periphery thereof can clearly be distinguished and visual information given by the target portion can easily be recognized in the case that an observer recognizes the target portion by the naked eye.
On the other hand, it is hard to sufficiently obtain the visibility of the marking if the intensity of the laser beam is controlled into a range in which the corrosion resistance is ensured. As a result, even if the technique disclosed in the Patent Document 3 is applied to the metallic parts having a concavo-convex to some extent through execution of processing such as the exterior cover described above, it has been hard to sufficiently obtain the visibility of the product-identifying information while ensuring a corrosion resistance.
Moreover, although the technique disclosed in the Patent Document 4 has a visibility ensured, it requires a thin film metal layer to be stuck onto a device, which is originally unnecessary itself, whereby the step becomes complicated, becoming a factor for a cost increase. Even if the same technique is applied to a metallic member, moreover, it is hard to ensure the visibility.